Auto transformer motor starter

 

Autotransformer starting is just a starter that will reduce the voltage at the start. When reducing that voltage we reduce that inrush current. In order to understand auto transformer starting you must first understand the basics of how an autotransformer works.

If you were to look at this autotransformer below, the general idea is that if I can bring the voltage down of my motor at the start, that will bring the current down of my motor. If I had a 300 volts source and used the 80% tap on the autotransformer, the voltage across the motor should be 240V.

When we apply 80% of the source voltage to the motor, we will only get 80% of the current.

That means that my current of the motor at the start is current at full voltage start times whatever tap (I(motor start)=I(full voltage start)xtap) I'm using, so in this case, I'm using the 80% tap (standard taps are 50%, 65%, and 80%), so I get 240 volts, if I was using 65 or 50, it would be different. But the line current is going to reduce even further and this is because of the rule with a transformer which is VA into the transformer has to equal VA out of the transformer. So if we have a VA out of our transformed here, because our voltage is higher here, our current gets to drop down even more, which means that our line current saw the current at the start is going actually to see a double reduction. So we can take our current full voltage start times our tap square

I(line start)=I(full voltage start)xtap²

hypothetically, if this was a 100amp start normally, it would be an 80 amp at the start and then 64 for the line, which means at the line terminals, we can use smaller conductors because of the small inrush current.

Material Needed:

Autotransformer: Used to reduce starting current in order to minimize high inrush current

Contactors: Used to energize and de-energize the motor

Control relay: Used to energize the contactor's coil in the circuit so it can remain energized once the momentary start push button is pressed.

Timer relay: The function of the timer relay is to switch the circuit from the autotransformer supply to the full voltage supply.

Overload relay (block): When a high current situation arises as a result of a ground fault, short circuit, phase failure, or mechanical jamming, overload relays turn off electricity to the motor by interrupting the flow of current to the contactor coil.

How does it work?

There are two sequences of operation about how this motor starter works, one called an open transition and a closed transition. With an open transition, we can get some torque bombs and current spikes. But, how an open transition works, it will completely remove itself from the circuit in the autotransformer start then it will start running on full voltage, and a closed transition will start running on full voltage, and then completely removes the autotransformer from the circuit

When the start push button is pushed it gonna creates a path for current to flow from line 1 to the control relay (CR) coil, once this happens the normally open control relay contact that's wired in parallel with the start push button is going to close then create another path for current to flow from line 1 to control relay (CR) coil, this is known as the holding contact which keeps the circuit energizes until the stop button push press, then-current start to flow from the next control relay contact to the timer relay (T.R) coil and contactor two (C2) coil from timer relay normally closed time open and contactor one (C1) normally closed. Then contactor two (C2) coil closes normally and opens the contactor two (C2) contact, therefore creating a path for current to flow to the contactor three coil (C3).

In this instance when contactor two is used to energize contactor three both contactors normally open the main contact is going to be closed, thus energizing the autotransformer and the autotransformer is then used to reduce the voltage going to the motor.

Once the timer relay (TR) reaches its preset time the normally closed time open contact is going to open, hence breaking the path for current to flow to contactor two (C2) coil which then opens contactor two (C2) normally open contact that supplies contactor three (C3) coil with current, therefore both contactor two (C2) and contactor three (C3) main contacts are open in this case so no current will be flowing to the autotransformer. The timer relay also closed the normally open time closed contact when the preset time is complete now current can flow to contactor one (C1) coil from normally open time closed contact (which is now close) and contactor two normally closed contact, during this state the motor receives its full voltage.

Note. In order to implement additional protection from the short circuit we use an electrical interlock in series with the contactor coils. Contactor two (2) coil is wired in series with a normally closed contact on contactor one (1) and contactor one (1) is wired in series with a normally closed contact on contactor two (2) to prevent both contactors from energizing at the same time.

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